The present invention relates to a golf club and, more particularly, to a golf club including a head of a hollow structure.
Hitherto, various contrivances have been made on conventional golf clubs of hollow structures so as to increase the flying distances of golf balls hit by the golf clubs. For example, JP-A-4-135576 discloses a golf club enabled to increase the flying distance of a golf ball hit with the golf club by reducing the rigidness of a crown portion of the golf club. According to this golf club, the rigidness of the crown portion is reduced, so that the crown portion bends at impact, and that a face portion is inclined to the border between the face portion and a sole, which serves as a supporting point. Consequently, a loft angle of the face portion increases to thereby reduce an amount of back spin due to a gear effect. Even in a case where the launch angle of this club is equal to those of other conventional clubs, the flying distance can be increased.
For example, JP-A-2004-187795 discloses a golf club configured so that a head of the golf club has a hollow structure, that a central part of a face portion is thick, while a peripheral part of the face portion is thin, and that a plurality of groove portions extending laterally are formed in an upper part than the central thick part of the face portion. According to this golf club, the deformation of an upper portion of the thick part at impact is larger than that of a lower portion thereof. A launch angle can be increased to a large degree. Consequently, the flying distance can be increased.
The golf club disclosed in JP-A-4-135576 is configured so that the entire face portion is inclined when hitting a ball. Accordingly, the loss of energy at hitting of a ball, the repulsion of a golf ball is lowered. Because the entire face portion bends, a launch angle cannot be so large. Consequently, there is a limit to an increase in the flying distance.
On the other hand, according to the golf club disclosed in JP-A-2004-187795, the groove portions are formed to thereby make an upper part of the face portion to easily bend, as compared with a lower part thereof. Thus, the launch angle can easily be increased. However, because the thin part is formed around the face portion, the entire face portion bends at impact. Thus, the loss of energy at hitting of a ball is large, so that the repulsion of the ball is lowered. Consequently, there is a limit to an increase in the flying distance.
Meanwhile, hitherto, various contrivances have been made on golf clubs, each of which is equipped with a head of a hollow structure, so as to increase the flying distance. It is known as one of the contrivances to cause the face portion to bend at hitting of a ball to thereby increase the flying distance. That is, when a ball is hit, the face portion bends to suppress the ball from being squashed. Consequently, the loss of energy due to the deformation of a ball is reduced. Increase in the flying distance can be achieved.
Thus, recently, a numerical value called Characteristic Time (here under referred to as a CT-value) has newly be en employed as an indicator for measuring a characteristic of the face portion of a head. The CT-value is a value measured in conformity with a pendulum test devised by the USGA (the United States Golf Association). An outline of the pendulum test is as follows. That is, a measurement apparatus disclosed in, for example, JP-A-2004-249086 is used and estimates the flexibility of a face portion by making a test piece to hit a central part of the face portion and by measuring a contact time between the face portion and the test piece. In a case where the CT-value is high, the face portion is easy to bend. The flying distance can be increased for the aforementioned reason.
A golf club disclosed in, for example, JP-A-2004-267438 is known as that designed by paying attention to such a CT-value. Further, JP-A-2004-267438 discloses a technique of shifting a maximum resilience point (i.e., a point at which the CT-value is highest) of a head from the position of the center of the face portion serving as a ball hitting surface of the head so as to form a golf club suitable for the swing form of each golfer. The head of a golf club is designed, especially, for a golfer, which is “liable to top a ball”, so that the maximum resilience point is positioned lower than the center of the face portion. Consequently, increase in the flying distance can be achieved.
The latest structure of the head has tended to increase the size thereof so as to stabilize the directionality of a ball and so as to increase the inertia moment of the head. The latest structure of the head has bee designed so that even when a ball is hit at a position that is off the face portion's center from which a maximum flying distance can be obtained when the ball is hit, a certain flying distance can be obtained.
Along with increase in the size of the head as described above, there is an increasing tendency of variation in speed component of a ball with a ball hitting position of the head. That is, considering a swing supporting point (i.e., a grip position) as a center, in an up-down direction (i.e., a crown-sole direction) of the head, inevitably, the speed component at a lower side (i.e., a sole side), which is more distant from the swing supporting point, is large.
Considering the shape of the face surface of the recent enlarged head, a roll is formed as seen from a side sectional view. The upper (i.e., the closer to the crown side) the ball hitting position on the face surface, more oblique impact is made against a golf ball. The lower (i.e., the closer to the sole side) the ball hitting position on the face surface, more front impact is made against a golf ball. Therefore, regarding impact efficiency, a lower-side part of the face surface has higher impact efficiency. More particularly, a loft angle of a head is defined as an angle between a tangential line at a point on the face portion and a line perpendicular to a reference horizontal plane. Due to the aforementioned roll, in a case where this point is lower than the central position of the face portion, the loft angle at the point is small. In a case where this point is upper than the central position of the face portion, the loft angle at the point is large. That is, it is considered that the closer to the sole side at a each point on which the loft angle is small, the ball hitting position on the face surface, a tendency of making front impact against a golf ball is increased. In view of impact efficiency, it is considered that the repulsion is enhanced.
Accordingly, it is considered that the initial speed of a ball hit by the lower part of the face portion is high due to a speed-component difference in the up-down direction of the head.
Meanwhile, when a ball is hit by a golf club, a golfer can sensuously grasp the flying distance of the ball according to the speed of the ball immediately after hitting the ball. Although it is difficult to determine an actual flying distance, for example, at golf driving ranges, and on uphill and downhill slopes on golf courses, a flying distance can be determined, to some extent, according to the initial speed of a ball at the hitting of the ball. Thus, in a case where the initial speed of the ball hit by a lower-side part of the face portion of a club is high as described above, a golfer is likely to make a mistake that he feels that the ball has been hit at a hitting point at which a maximum flying distance can be obtained by the club.
However, as described above, the recent enlarged head of a club has a structure configured so that the maximum flying distance of a ball hit by the club is easily obtainable when the ball is hit by the central part of the face portion of the club. More particularly, in consideration of the relationship between the face portion and the gravity center of the head, the structure is such that the maximum flying distance of a ball hit by the club is easily obtainable when the ball is hit by a part which is slightly upper than the central part of the face portion of the club. Accordingly, the maximum performance of the club cannot fully be used by a golfer who makes a mistake that he feels that the lower-side part of the face portion of the club is an optimum hitting point at which a maximum flying distance can be obtained by the club. Consequently, the golfer recognizes the golf club as being unable to obtain a long flying distance. Thus, increase in the flying distance cannot be achieved.
Additionally, it is considered that the speed of a ball at hitting thereof is increased by designing a head so that the maximum resilience point is positioned lower than the center of the face portion, as disclosed in the JP-A-2004-267438. However, in the case of taking the shape of the head into consideration, when a ball is hit by a lower-side part from the center of the face portion, there is a limit to increase in the flying distance. That is, in the case of hitting a ball by a lower-side part from the center of the face portion, an amount of back spin is too large, due to reduction in the launch angle because of the shape of the roll of the face surface and to the gear effect caused by deviation in the up-down direction of the head around the gravity center at hitting of a ball. From a total standpoint, there is a limit to increase in the flying distance.